1. Field of the Invention
The present invention relates generally to methods and apparatus for perforating wells and particularly to actuators for actuating the firing heads of perforating guns.
2. Description of Related Art
In the completion of an oil or gas well, the casing of the well is perforated to communicate the well bore with the hydrocarbon producing formation which is intersected by the well. After the well has been drilled and cased, a perforating gun with shaped charges is lowered into the well to a location adjacent the hydrocarbon producing formation. A firing head associated with the perforating gun detonates the shaped charges which penetrate the casing thus allowing formation fluids to flow from the formation through the perforations and into the production string for flowing to the surface.
Many techniques have been used in the past to actuate perforating guns and perforate the casing. For example, perforating guns have been actuated electrically, through drop bar mechanisms, and through pressure actuation.
Historically perforating guns have been actuated electrically. The firing head and perforating gun are lowered into the well on a wireline. Electrical current is sent through the wireline to set off the firing head which in turn detonates the shaped changes in the perforating gun.
Other techniques are employed in tubing conveyed perforating systems. In such a system, the firing head and perforating gun are lowered into the well on the end of a tubing string. One method of setting off the firing head is to drop a weight through the bore of the tubing string to impact the firing head and detonate the perforating gun. Tubing conveyed perforating systems are available from the Halliburton Company, the Assignee of the present invention.
Other tubing conveyed perforating systems employ a differential firing head which is actuated by creating a pressure differential across an actuating piston in the firing head. The pressure differential is created by applying increased pressure either through the tubing string or through the annulus surrounding the tubing string to move the actuating piston in the firing head. Typically, the firing head actuating piston will have hydrostatic pressure applied across the actuating piston as the tool is run into the well. When it is desired to operate the tool, the increase in pressure is sufficiently large to initiate detonation of the firing head and perforating gun. Thus, hydrostatic pressure is on the low pressure side of the actuating piston and the increased pressure in the tubing string or annulus is on the high pressure side of the piston.
A commercially available firing head system is the VannJet.RTM. firing head and differential firing head combination manufactured and sold by the Vann Systems Division of Halliburton Company. In this system, the firing head and perforating gun are again lowered on a tubing string. This firing system includes a stinger which protrudes upwardly within the tubing string from above the differential firing head. A first explosive pathway extends from the upper end of the stinger to the firing head. The first explosive pathway includes a first booster charge, a length of primacord and a second booster charge. The VannJet.RTM. firing head is lowered through the tubing string on a wireline and received over the stinger. A pressure increase within the bore of the tubing string is applied to the VannJet.RTM. assembly causing the VannJet.RTM. actuator to initiate a percussion detonator which in turn initiates the first explosive pathway. Alternatively, the VannJet.RTM. firing head might be actuated by mechanical jarring or use of an electric timer. Methods which depend upon pressure increases transmitted down the tubing string or annulus from the surface have disadvantages. Quite often, required actuating pressures approach the pressure safety limits for surface equipment. These methods cannot be used in wells which have already been perforated since the previous perforations bleed off the increased pressure into the formation.
Further, such methods are cumbersome for perforating in an underbalanced condition, wherein the annulus pressure is lower than the formation pressure during detonation of the perforating gun. In practice, movement of the actuating piston often requires large and costly amounts of injected nitrogen to generate the needed pressure differentials. If annulus pressurization is used to initiate detonation, delay timing, using for example, pyrotechnic or electrical time delays, is necessary to allow the pressure to be bled off the annulus prior to detonation.
One technique which avoids having to pressurize the tubing string or annulus is use of an electronic timer to operate an electrically-actuated blasting cap inside the combined firing head and gun. After the gun has been placed in the well, the timer is set preset to expire after a predetermined amount of time and then lowered by slickline into the tubing string to contact the basting cap in the gun. When the timer expires, an electric current is transmitted to the blasting cap detonating it. This system poses a safety risk since the electrical blasting cap is prone to premature detonation caused by stray electricity prior to being run into the well. Also, if the gun fails to fire at the end of the predetermined amount of time, the gun cannot be safely retrieved because of the risk of a delayed detonation of the cap following removal of the gun from the well. An appropriate backup detonation system, such as those disclosed in U.S. Pat. No. 5,301,755 to George, et al. and assigned to the Halliburton Company, would have to be used to ensure detonationg of the gun.
Another technique which avoids pressurization is described in the George patent. A differential firing head is mounted on the perforating gun and lowered into the well on a tubing string. A landing nipple disposed in the tubing string above the differential firing head forms a lower tubing bore with the firing head. The differential firing head includes an actuating piston having a high pressure side communicating with the wellbore annulus through ports and a low pressure side communicating with the lower tubing bore. The annulus pressure and lower tubing bore pressure are substantially the same as the firing head and perforating gun are lowered into the well such that the pressure across the actuating piston is balanced. A firing head actuator is lowered through the tubing string and seated in the landing nipple above the differential firing head. The firing head actuator includes an atmospheric chamber with a valve for opening the atmospheric chamber to the lower tubing bore. The firing head actuator also includes an electric timer connected to a control system for opening the valve and thus exposing the atmospheric chamber to the lower tubing bore. The electric timer is preset to allow a predetermined amount of time to pass before the valve is opened. Upon opening of the valve, fluid trapped at hydrostatic pressure within the lower tubing bore is allowed to flow into the atmospheric chamber. Unbalanced pressure across the actuating piston of the firing head causes the actuating piston to move and actuate the differential firing head and perforating gun. The firing head actuator allows the well to be in an underbalanced condition during actuation since pressure increases are not used to start actuation.
In practice, hydrostatic pressures of about 2,000 psi have been required to operate the actuator of the '755 patent making the design functional in most, but not all, cases, without annulus pressurization. This hydraulic arrangement for detonation of the gun places the actuator into proximity with the perforating gun rather than creating a direct explosive pathway between the actuator and firing head. This arrangement provides a measure of safety since the actuator is not directly associated with the firing head and perforating gun charges until the actuator is placed downhole.
It would be desirable, then, to have an actuating system which is useful for detonating the gun in underbalanced and other wellbore conditions. The system should afford the relative effectiveness and certainty of systems which provide a complete explosive pathway between the actuator and the gun while maintaining the safety of proximity systems which keep the actuator separate from the gun at the surface and during emplacement. It would also be desirable to have an actuating system which did not require application of wellbore pressurization in order to operate reliably and which allows use of back up detonating systems.